The increasing availability of information in the form of data from various sources has lead to a large public demand for data transfer that challenges the capabilities of existing communication delivery systems. A wide variety of types of information sources are available in the form of discrete devices and systems, as well as tools available over shared sources such as the Internet and private data network sources. The associated end-use applications for such information is also diverse and including industrial, commercial and retail markets, including for example personal computers, full-motion video devices, cellular communication devices, and PDAs. The technological development has grown largely due to expanded used and demand for a greater range of subject matter content and delivery of such content at higher speeds.
Another aspect challenging development of such high-performance devices is the need to reduce power consumption. In connection with portable devices, the cost and limited usage time provided by batteries has led to an ongoing effort to significantly reduce power consumption without compromising size or performance. With more recent concerns regarding power availability, power consumption concerns are no longer limited to portable devices.
Presently there exists a great demand for wireless systems that achieve high data transmission rates while using as little power and bandwidth as possible. Maximum data transfer for a given channel width demands sophisticated modulation techniques, the best of which require a linear power amplifier (PA). The strong tradeoff between linearity and power efficiency in PA's has motivated research into linearization techniques, of which Cartesian feedback is an important and promising example.
The forward path of a typical Cartesian feedback system includes separate gain and filter elements for each Cartesian component (I and Q). Immediately following these elements is a quadrature modulator, whose output in turn drives the power amplifier. Feedback is provided by coupling a signal from the output of the RF amplifier to a quadrature demodulator. In general, it is necessary to provide a phase shift between the local oscillator inputs of the modulator and demodulator.
The adjustment of the phase shift signal ensures synchronous demodulation of the baseband signal. Properly adjusted, the system functions as two decoupled feedback loops: one for the I channel, and one for the Q channel. Feedback stability margins degrade as this adjustment departs from the optimum, and instability can result. The exact phase shift required can drift over time, temperature, and process variations, and usually changes with carrier frequency, which is particularly troublesome for frequency-hopping systems. To allow for linearization at the maximum symbol rate, this phase shift must be regulated as accurately as possible. In addition, rejection of drift with temperature demands continuous regulation. Thus far, these problems have been mitigated to a limited extent using non-exact approximation techniques and relatively complex digital signal processing.